Diaphragm cells useful for the electrolysis of brines and the formation of chlorine and caustic soda have an anolyte chamber and a catholyte chamber. The anolyte chamber contains an anolyte solution of sodium chloride at a pH of from about 3.0 to about 4.5. Inside the anolyte chamber is an anode at which chlorine is evolved. The catholyte chamber of a sodium chloride chlor-alkali cell contains from about 10 to 15 weight percent sodium hydroxide while the catholyte chamber of a potassium chloride chlor-alkali cell may contain as much as 25 weight percent potassium hydroxide. The catholyte liquor contains the alkali metal hydroxide, and the alkali metal chloride. Alkali metal hydroxide is formed in the catholyte and hydrogen gas is evolved at the cathode.
In the operation of sodium chloride diaphragm cell, brine containing approximately 300 to 315 grams per liter of sodium chloride is fed into the anolyte chamber. At the anode, the reaction 2Cl.sup.-.fwdarw. Cl.sub.2 + 2e.sup.-
Takes place.
The anolyte liquor passes from the anolyte chamber through the diaphragm into the catholyte chamber, and a catholyte product containing from approximately 110 to approximately 150 grams per liter of sodium hydroxide and from approximately 120 to approximately 200 grams per liter of sodium chloride is recovered. In the catholyte chamber, the reaction 2H.sub.2 O + 2e.sup.-.fwdarw. 20H.sup.-+ H.sub.2 takes place.
Typically, diaphragms for chlorine cells have been prepared from asbestos. Most commonly, the asbestos is chrysotile asbestos. The chrysotile asbestos is used to provide a diaphragm having a thickness of from about 1/8 inch to about 1/4 inch. Chrysotile asbestos diaphragms generally have a service life of about 6 months.
It is necessary to periodically remove the diaphragm cell from service in order to remove the old diaphragm from the cathodes, and install a new diaphragm on the cathodes. This periodic removal of the cell from service results in a consequent loss of production in order to carry out a labor-intensive cell renewal operation. In the past, when diaphragm cells used graphite anodes, diaphragm renewal could be coordinated with anode renewal. However, metallic anodes, e.g., coated titanium anodes, have replaced graphite anodes to a considerable degree. While graphite anodes have a service life of from about 4 to about 8 months, coated metal anodes have a longer life, e.g., 3 to 4 or more years. Accordingly, the renewal of the diaphragms has become a principal factor in cell outage.